This invention relates to mud pulsers for creating pressure pulses in the flow of mud.
Mud pulse telemetry systems are well known in the oil well drilling art. Mud pulse telemetry systems transmit information through a flowing column of drilling mud. In this process the pressure in the flowing mud column at a point downhole is periodically modulated by a fluid valve. As a result, periodic pressure pulses appear in the mud which travel uphole in the surface and are detected by a pressure transducer. Information may be conveyed by the presence or absence of pulses through use of various digital and/or analog encoding and decoding techniques.
Various types of hydraulically operated valves are used for momentarily restricting the flow of mud to thereby create pulses in the mud. For example, one type of hydraulic valve involves a poppet and orifice in which the poppet is actuated toward the orifice in order to momentarily restrict the flow of fluid. In another type of device a tubular sleeve is provided through which the mud is forced to flow. The increased pressure in a separate fluid around the exterior of the sleeve causes the sleeve to decrease in interior diameter, thereby momentarily restricting mud flow and increasing pressure, thereby creating a pressure pulse in the mud flow.
An example of the last mentioned device in U.S. Pat. No. 2,898,088 in the name of Alder. This device has upper, middle and lower fluid chambers for fluid which is separate from the mud flow. Each chamber surrounds a different sleeve. The upper and lower chambers form upstream and downstream pressure chambers, and the middle chamber forms a valve for restricting the flow of mud. An electrically operated value is provided for switching the separate fluid from the upper chamber to the chamber around the sleeve in the valve. The pressure around the sleeve in the valve restricts the flow of mud therethrough, creating a pressure pulse in the mud. The electrical valve when deactuated allows the fluid, which has been transferred from the upstream reservoir to the chamber around the sleeve of the valve, to be transferred to the downstream reservoir. A pump is provided for pumping the fluid from downstream reservoir back up to the upstream reservoir.
A serious disadvantage of the latter arrangement is that severe restrictions are placed on the design of the electrically operated valve. If it is desired to operate the flow restriction valve rapidly, then the electrically operated value must be designed with very large ports for switching the separate fluid. If the ports are made large then a large amount of electrical power is required for operating the valve. If it is desired to reduce the electrical power required to operate the electrically operated valve, then it is necessary to reduce the sizes of the orifices in the valve which in turn reduces the speed with which the flow restriction valve may be operated.
Flow restriction valves for creating pulses in flowing mud are also known which employ poppets which are operated by solenoids, clutches and other means. By way of example such a device is referenced in the Oil & Gas Journal, May 29, 1978, in an article entitled "System is Available for Measuring Hole Direction" (pp. 69-76) and in U.S. Pat. No. 2,759,143.